The field of the invention is, generally speaking, that of a digital photograph and, more specifically, that of the image processing applied to digital photographs. By digital photograph is meant a photograph obtained as a result of printing a digital image. Generally speaking, a digital image is comprised of pixels, each pixel designating a position and a portion of the image space, each pixel having at least a pixel value. The images stemming from all photographic apparatus, called “photographic images”, are images having a dimension two, within the meaning of the image space as a rectangular portion of a plane. However, images, notably stemming from physical or medical observations, may have increased dimensions, notably three. Likewise, a video stemming, for example, from a camcorder, may be considered as a succession of images with a dimension two, or as a single image with a dimension three: the first two dimensions being, in this case, the dimensions of the images and the third dimension representing the image number within the video.
A digital image is acquired using a digital capturing apparatus. Generally speaking, a digital capturing apparatus is an apparatus comprising at least one sensor and one optical system, adapted to obtain a digital image. A sensor is a system converting light into digital values reflecting the color intensity of each point of the digital image. A sensor can notably contain a photosensitive receiver enabling to convert the photon energy into electric energy. Within a standard sensor structure, such photosensitive receiver can be defined as a set of photosites. The photosites can notably be associated with a particular color like, for example, in the case of a Bayer structure containing two green photosites, a red photosite and a blue photosite. Generally speaking, an optical system is a set of lenses, mirrors and transparent and homogeneous media separated by dioptres.
Within the field of digital photography, due to the digitalization of the photographic manufacturing chain, as from its capture to its reproduction on paper, it is more and more frequent to propose image-processing solutions capable of removing certain apparent faults on a large number of digital images acquired using digital capturing apparatus. For example, a very frequent fault is the one known as the “red-eye” phenomenon. Other problems relating to the use of photographic apparatus exist, whether they are in fact digital or not. Hence, for example, distortion problems are frequently encountered. Generally speaking, a known distortion phenomenon is the geometric distortion, characterized like a geometric aberration and in the presence of which the object points, aligned within the photographed scene, do not form aligned image points. Such geometric distortion is characterized by an image-enlargement modification depending on the position within the field. Two types of distortions can be distinguished: cylinder-shaped distortions and cushion-shaped distortions depending on whether the local enlargements diminish or increase when nearing the edge of the field.
Another problem encountered is known under the name of lateral and longitudinal chromatic aberrations. Generally speaking, known chromatic aberration phenomena concern the lateral and longitudinal chromatic aberrations, such as a geometric aberration in the presence of which the image enlargement varies according to the wavelength. In the context of lateral chromatic aberration, the image is shaped on the same plane, whatever the wavelength, although its position on the plane depends upon the wavelength. In the context of longitudinal chromatic aberration, the position of the focusing plane varies according to the wavelength, which generates a different blur depending on the wavelength. The chromatic aberration generates colored irisations in the vicinity of the image transitions. Various solutions have been proposed in the state of the art in order to deal with such problems.
Another known problem in the field of photography, and notably that of digital photography, is a problem designated by the relative illumination phenomenon expression.
Relative illumination refers to the phenomenon inducing a variation of the pixel values on a digital image according to the positions of the pixels in the field. For example, for a digital image of a homogeneous scene, the relative illumination phenomenon corresponds to a variation of pixel values according to their position in the field. It becomes noticeable by a darkening of the corners, or even by a loss of data. FIG. 1 shows such type of relative illumination phenomenon: here a curve 10 represents a darkening phenomenon, graded on the axis of the ordinates, according to a position within the image, relating to a reference point, graded on the axis of the abscissa.
Several relative illumination sources can be identified for an image stemming from an apparatus:                A lens is characterized by its field angle and by its field coverage. The sources are calculated so as to cover a given photosensitive receiver format. The relative illumination is largely due to a window effect inside the lens. A full beam parallel to the axis entering the lens is transmitted, whereas an oblique beam will be partially stopped by the lens mount, so as not to be transmitted beyond a borderline angle. If the lens is designed to form a field of full light having the dimensions of the photosensitive surface for a certain aperture value, it is possible that when fully open, the surface of the receiver exceeds that of the field in full light, which will cause a darkening of the corners of the image. There is a relative illumination phenomenon when the field coverage is lower than the format of the sensitive surface.        Within the field covered by a lens, the lighting is not rigorously uniform in accordance with the rays comprising its image due to the simple fact of photometry and geometry. The further the image away from the axis, the more the relative illumination phenomenon is enhanced.        The digital sensors, in light of their design, provide a variable response depending on the incidence of the rays affecting it. The further the image away from the axis, the more the relative illumination phenomenon is enhanced.        The relative illumination phenomenon can be caused by the obstruction of the image ray through a body of the apparatus, or by an accessory placed within the lens field. This may, for example, be due to the mount of a filter, or more commonly to an unsuitable sun-shade.        
The relative illumination phenomenon can be measured on an image stemming from an apparatus having an exposure rating, the relative illumination value measuring the exposure-rating difference between the quantity of light received by the edges of the digital image and that received at the center of the digital image.
Solutions to such relative illumination phenomenon have been proposed in the state of the art.
Nevertheless, the appearance of new sensors intervening inside the digital image-capturing apparatus gives rise to a new problem, designated as the colored relative illumination. The colored relative illumination refers to the phenomenon inducing a variation of the pixel values on a digital image according to the positions of the pixels within the field depending on the digital image color channels. An image can be decomposed into channels in many ways, having an image pixel thus corresponding to a pixel value for each one of the channels. In the particular case of the color images, the decomposition into channels, such channels thus being called “color channels”, can notably be performed using a decomposition inside the RGB, sRGB, LMS, Lab, Yuv, HSL, HSV, XYZ, xyz color spaces. Such terms, with the exception of sRGB, are defined, for example, in the following publication: “Measuring Color”, Third Edition, R. W. D. Hunt, Fountain Press, Kingston-upon-Thames, England 1998, ISBN 0863433871, or in the publication “Color Appearance Models”, M. Fairchild, Addison Wesley, 1998, ISBN 0201634643. The sRGB color space is described in the IEC standard 61966-2-1, “Multimedia systems and equipment—Colour measurement and management—Part 2-1: Colour management—Default RGB colour space & sRGB”. In the context of the invention, the digital image may also be constituted of one or several channels relating to other data having no relation to the color like, for example, the digital values relating to the physical dimensions, notably altitude, distance and temperature.
In the case of a digital image, observation of the colored relative illumination phenomenon is, for example, the relative illumination measurement in exposure rating on each color channel. The exposure ratings are not all identical in the case of a colored relative illumination phenomenon. FIG. 2 shows such colored relative illumination phenomenon: here a first curve 20, respectively a second curve 21, represents an exposure rating for a first color channel, respectively for a second color channel, graded on the axis of the ordinates, according to a position within the image, relating to a reference point, graded on the axis of the abscissa. As observed, the colored relative illumination phenomenon varies from one color channel to another. A colored relative illumination phenomenon is represented here, varying within a single dimension, for example, a distance to a point. In another embodiment, such colored illumination phenomenon varies according to several dimensions of the image.
By light source is meant a physical transmitter of visible energy. Examples of light sources are the sky at different times of the day, neon lighting, and a tungsten bulb. An illuminant represents the spectral distribution of a light source. A spectral distribution represents a radiometric quantity according to the wavelength. Such radiometric quantity can notably be the spectral exposure in the case of a light source, or the result of the spectral reflection factor from the material by the spectral distribution of a light source in the case of a material reflecting the light, or the result of the light transmittance from a material by the spectral distribution of a light source in the case of a material transmitting light. Some illuminant examples are: the CIE A illuminant representing the source of light in the form of a Planck radiator having a color temperature of 2856 K, the CIE D65 illuminant which is a statistic representation of the average daylight corresponding to a color temperature of approximately 6500 K, the CIE C illuminant, the CIE E illuminant, the CIE D illuminants (including the CIE D50 illuminant), the CIE F illuminants (including CIE F2, CIE F8 or CIE F11). Such terms and other illuminant examples are defined, for example, in the following publication: “Measuring Color”, Third Edition, R. W. D. Hunt, Fountain Press, Kingston-upon-Thames, England 1998, ISBN 0863433871, or in the following publication, “Measuring Color”, Third Edition, R. W. D. Hunt, Fountain Press, Kingston-upon-Thames, England 1998, ISBN 0863433871, or in “Color Appearance Models”, M. Fairchild, Addison Wesley, 1998, ISBN 0201634643.
FIG. 3 illustrates that the colored relative illumination phenomenon, for a given color channel, varies for two different illuminants. In this figure, a first curve 30, respectively a second curve 31, represents a relative illumination phenomenon for a first type of illuminant, respectively for a second type of illuminant, graded on the axis of the ordinates, according to a position within the image, relating to a reference point, graded on the axis of the abscissa. As observed, the colored relative illumination phenomenon varies from one color channel to another. A colored illumination phenomenon is represented here, varying within a single dimension, for example, a distance to a point. In another embodiment, such colored illumination phenomenon varies according to several dimensions of the image.
A given illuminant is associated with a spectral distribution of the light source depending on the wavelength, as illustrated in FIGS. 4 and 5, showing respectively a first curve 40 providing the spectral distribution of a first illuminant, and a second curve 50, providing the spectral distribution of a second illuminant.
The photon energy quantity of the incident light on the digital sensor may vary according to the illuminants, as illustrated in FIGS. 6 and 7, showing respectively:                a first curve 60 providing the amplitudes of the signals transmitted to two different wavelengths for a first illuminant, under the same angle of incidence, for a given sensor.        a second curve 70 providing the amplitudes of the signals transmitted to two different wavelengths, the same as those shown in FIG. 6, for a second illuminant, under the same angle of incidence, and for the same given sensor.        
The quantity of photon energy depending on the wavelength can be modified according to the specifications of the optical system, such as the existence of filters able to modify the energy transmitted depending on the wavelength of the incident light on the filter.
A digital sensor has a spectral response: the amplitude of the digital values reflecting the intensity of the color channels of each point of the digital image depends upon the wavelength of the incident light on the sensor. The light angle of incidence on the sensor notably depends upon the specifications of the optical system, such that an optics authorizing the high chromatic aberrations or even the value of the average ray angle, i.e. the angle between the optical axis of the optical system and the ray passing through the summit of the object and the center of the pupil.
A digital sensor does not emit the same response on each part of the photosensitive receiver depending on the angle of incidence of the rays affecting it, as illustrated in FIG. 8, such Figure showing a first curve 80 and a second curve 81 which respectively corresponds to a first spectral response of a given sensor for a first angle of incidence i1, and to a second response given by the same sensor for a second angle of incidence i2.
The reasons for such a phenomenon can be:                asymmetries in the physical constitution of the sensor: photosites having different implantation angles on the sensor;        various micro-lenses according to the photosites, the micro-lenses being optical systems designed to focus the incident rays on the digital sensor in the direction of the photosensitive receivers that are the photosites for improving the result thereof;        different wiring depending on the photosites;        different 3D geometry according to the photosites, for example due to the existence of transistors or of non-symmetric connections;        different physical implantations within the field;        different geometry for adjacent photosites;        characteristics of an IR filter, i.e. a filter especially allowing the passing of light, the wavelength of which being comprised between, for example, 425 and 675 nm. Depending on the orientation of the rays, the light transmittance properties of the filter may vary by several tens of nm, inducing a different behavior at color level in the corner and at the center of the image; and        different orientations according to the color of the photosites.        
The greater the angle, the more it is possible that the incident rays do not affect the most sensitive zone of the sensor, or are stopped, for example, by the opaque shields which cover the transfer registers. Generally speaking, the response of the sensor may decrease when the angle of incidence increases. The image of a point is not formed by a ray, but by a beam of rays whose incidence is comprised between two borderline values. The many incidences comprising such beam will be differently interpreted by the sensor, i.e. the energy of the incident light on the photosensitive receivers will not be the same for all wavelengths, and this, even for the adjacent photosites.
The adjacent photosites may represent different color channels. In each photosite to which at least one geometric position on the sensor corresponds, in addition to physical specifications such as described above, the amplitude of the signal notably depends upon the spectral response of the sensor in accordance with the wavelength of the incident light on the photosite and with the angle of incidence of such light, the photon energy of such light depending upon the illuminant and the specifications of the optical system. The colored relative illumination is linked to the illuminant and to the pixel value in each color channel of the digital image's pixels, the pixel values depending upon the amplitude of the signal of a certain number of the sensor's photosites.
All such phenomena and considerations entail the appearance of a relative illumination phenomenon, for example, a colored relative illumination on the photographs taken using digital image-capturing systems. The quality of the photographs is thus affected.